Are rotational DoFs essential in substructure decoupling?

Substructure decoupling consists in the identification of the dynamic behavior of a structural subsystem, starting from the known dynamic behavior of both the coupled system and the remaining part of the structural system(residual subsystem). The degrees of freedom (DoFs) of the coupled system can be partitioned into internal DoFs (not belonging to the couplings) and coupling DoFs. In direct decoupling, a fictitious subsystem that is the negative of the residual subsystem is added to the coupled system, and appropriate compatibility and equilibrium conditions are enforced at interface DoFs. Compatibility and equilibrium can be required either at coupling DoFs only (standard interface), or at additional internal DoFs of the residual subsystem (extended interface), or at some coupling DoFs and/or some internal DoFs of the residual subsystem (mixed interface). Using a mixed interface, rotational coupling DoFs could be eliminated and substituted by internal translational DoFs. This would avoid difficult measurements of rotational FRFs. This possibility is verified in this paper using simulated experimental data.